Dilute caustic alkali solutions, for example sodium hydroxide and potassium hydroxide solutions, are used in a wide range of industrial processes, for example for cleaning bottles and glass pipelines in the diary, soft drinks and brewery industries, in paper pulping processes, and in the scouring and bleaching of textiles.
Such dilute caustic alkali solutions, which may contain from, for example, 0.5 to 5% by weight of the alkali metal hydroxide, are usually prepared by dilution of the concentrated solutions containing perhaps 50% by weight of alkali metal hydroxide supplied by the manufacturer, or possibly by dissolution of the alkali metal hydroxide in solid form.
Exceptionally a process may require the use of a deionized or softened water for the dilution or dissolution, but for most processes, the use of such water is not feasible economically, and ordinary municipal tap water is used. It has been proposed, in order to stabilize such solutions, to add aminotri(methylenephosphonic acid). Diethylenetriaminepenta(methylenephosphonic acid) is another known aminophosphonic acid derivative which functions as a precipitation inhibitor under certain conditions, but which is almost totally ineffective in alkaline solutions at sub-stoichiometric dosages.